Ohmic contact to p-type InP

ABSTRACT

An ohmic contact to a semiconductor device comprising p-type InP is formed by sequentially depositing beryllium-gold and gold layers on InP and then heat-treating the device at a temperature less than 440 degrees C. An ohmic contact to p-type InGaAsP can be similarly formed.

TECHNICAL FIELD

This invention is concerned with a method of making ohmic contacts top-type InP and InGaAsP and to semiconductor devices having suchcontacts.

BACKGROUND OF THE INVENTION

Successful semiconductor device fabrication and operation frequentlyrequires contacting the semiconductor device with low resistance ohmiccontacts. Problems often arise in attempting to fabricate and use suchcontacts. For example, the contacting material may form a rectifying,rather than ohmic, contact with the semiconductor material, or it maynot reliably bond to the semiconductor material, and physicallyunreliable electrical contacts result.

Group III-V semiconductor compounds are of much interest today, and mucheffort has been directed toward developing reliable ohmic contacts withsuch compounds. Many processes for fabricating low resistance ohmiccontacts to such compounds are known. These processes typically involvethe deposition of one or more layers and may or may not involve one ormore heat treating steps. U.S. Pat. No. 3,214,654 describes ohmiccontacts to Group III-V compounds which are formed by a layer of a metalselected from the group consisting of silver, gold, ruthenium, rhodium,palladium, osmium, irridium and platinum and a layer of either nickel orcobalt. Germanium-palladium contacts to n-type Group III-V compounds aredescribed by U.S. Pat. No. 4,011,583.

Particular interest has recently been shown in Group III-V compoundsthat are useful in optical devices, such as light emitting diodes,lasers and photodetectors, that operate at wavelengths longer than 1.00micron. It should be understood that the term "light," as used in thisspecification, includes both the visible and the near infrared portionsof the electromagnetic spectrum. Interest in devices that operate inthis region has arisen primarily because the silica-based optical fibercompositions presently contemplated for optical communication systemshave smaller material dispersion, as well as low loss, above 1.00 micronthan they do below 1.00 micron.

One class of light emitting devices presently contemplated for suchsystems uses the quaternary alloy, InGaAsP, which is grown on InP. Suchdevices are useful between 0.95 μm and 1.68 μm. These light emittingdevices operate at high forward current and require high quality ohmiccontacts to reduce series resistance. For this class of device, as wellas others, ohmic contacts to InP are necessary.

While low resistance ohmic contacts to n-type InP can now be easilyfabricated, the formation of ohmic contacts to p-type InP still presentsdifficulties. P-type contacts to InP have been made using Zn as theacceptor. While these contacts are quite acceptable for many purposes,they have a number of drawbacks. For example, Journal of AppliedPhysics, 46, pp. 452-453 (1975) reports a rather high resistance,namely, 10⁻³ ohm.cm², for an electroplated Au/Zn/Au metallization.Furthermore, additional problems arise when Zn is used as the acceptorbecause the relative volatility of Zn makes it difficult to fabricatethe contact with vacuum deposition techniques. Moreover, rapid diffusionof the Zn through the InP, together with the high doping concentrationsrequired, may cause either junction motion or long-term devicereliability problems or both.

Ohmic contacts to some Group III-V compounds using Be-Au metallizations,i.e., Be is used as the acceptor, are known. For example, suchmetallizations have been made to p-type GaP. However, formation of theseohmic contacts has required heating the GaP devices to the relativelyhigh temperature of 600 degrees C. for approximately 5 minutes to formthe ohmic contact. Alloying temperatures of 600 degrees C. cannot beused to form ohmic contacts to either InP or InP containing devicesbecause InP begins to decompose through P outdiffusion at approximately400 degrees.

SUMMARY OF THE INVENTION

We have found that ohmic contacts having low resistance can be made top-type InP material in a semiconductor device by using beryllium as theacceptor. The contact is formed by sequentially depositing, on the InP,a 1 to 3 percent, by weight, beryllium in gold (Be-Au) composition and agold overlay. The deposition is followed by heat treating the depositedmaterial at a temperature less than 440 degrees C. for a time of atleast 1 minute. Deposition of a palladium layer on the InP layer priorto the deposition of the Be-Au layer permits use of a heat treatingtemperature less than 420 degrees C. but generally results in a contactwith a slightly higher resistance. This method may also be used toproduce a low resistance ohmic contact to p-type InGaAsP.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional view of a device processed according to thisinvention.

FIG. 2 plots alloying temperature, horizontally, versus resistance inohms, vertically, for a contact of this invention; and

FIG. 3 plots alloying time, horizontally, versus resistance in ohms,vertically, for a contact of this invention.

DETAILED DESCRIPTION

FIG. 1 shows a semiconductor device 1 having a layer 10. Layer 10 may bea substrate, but is more typically an epitaxial layer grown on asubstrate. Device 1 may be a light emitting diode, laser, etc. Layers20, 30 and 40 are sequentially deposited above layer 10 and form theohmic contact after heat treating. The semiconductor device furthercomprises additional semiconductor materials (not shown) deposited onlayer 10 opposite layer 20. Layer 10 consists of InP. Layer 20 consistsof palladium, layer 30 consists of a beryllium-gold composition, andlayer 40 consists of gold. For reasons that will be explained, thepresence of layer 20 is optional. If layer 20 is omitted, layer 30 isdeposited directly on the substrate. The InP layer may be covered withan InGaAsP layer prior to deposition of layer 20, in which case theohmic contact is made to the InGaAsP layer. Conventional techniques,such as electron gun evaporation, may be used to deposit the layers.Beryllium has a vapor pressure very similar to that of gold and can,therefore, be evaporated very reproducibly from beryllium-gold sources.Pressures are desirably held below 6×10⁻⁶ torr.

Conventional p-type dopants may be used in the InP substrate. Forexample, Zn, with a concentration of 8×10¹⁸ cm⁻³ may be used in aliquid-encapsulated Czochralski (LEC) grown substrate. The particularp-type dopant used is not critical to formation of an ohmic contact withthis invention. The dopant concentration should, however, be at least10¹⁷ cm⁻³ to form an ohmic contact. The dopant concentration should beas high as is practical because resistance decreases as the dopantconcentration increases. The method of substrate growth and thesubstrate orientation are both noncritical.

Layer 20 is optional and when present, is approximately 100 A thick. Alayer of 100 A is sufficiently thick to trap outdiffusing P throughformation of intermetallic P-Pd compounds without impeding Be migrationinto the InP substrate. Thicker layers may result in the formation ofundesired Pd compounds. This layer permits, as subsequently described,lowering of the heat treating or alloying temperature and, therefore,reduction of the InP tendency for thermal dissociation. There may,however, be a slight increase in contact resistance when the Pd layer ispresent.

Layer 30 consists of a gold beryllium composition having between 1 and 3percent, by weight, beryllium. The described weight percent range of Beis desirable because Be and Au form well-defined structures within thisrange. Layer 30 is typically 800 A thick, although thicknesses as smallas 600 A and as large as 1000 A may be used. Below 600 A, there may notbe sufficient Be for the reaction, and above 1000 A, too much Be may bepresent. The presence of too much Be makes contact formation difficultas the reaction is driven by Au. A Be content of 3 percent is preferredover 1 percent because at the lower weight percent, contact uniformly isnot as good.

Gold layer 40 is at least 2100 A thick and may be thicker if so desired.However, if layer 40 is thinner, the contact may not be uniform andsmooth after heat treating. The minimum thickness is conveniently used.

After deposition of the layers, the structure is heat-treated at atemperature less than 440 degrees C. for a residence time of at least 1minute. If the palladium layer is not present, the preferred range forheat treating is between 400 degrees C. and 440 degrees C., and theresidence time is between 5 and 10 minutes. If palladium layer 20 ispresent, the alloying temperature is preferably less than 420 degreesC., and the residence time is at least 1 minute. The preferred heattreating temperature is approximately 400 degrees C. Temperaturesoutside the above range have higher contact resistances, and are,therefore, less prefered.

Heat treating conveniently takes place in any of the conventionally usedatmospheres such as forming gas (a hydrogen-nitrogen mixture), argon ornitrogen.

Alloying or heat treating times and temperatures may be determined withmore specificity by reference to FIGS. 2 and 3 which show the measuredresistance as functions of treating temperatures and times,respectively.

FIG. 2 plots alloying temperature in degrees centigrade, horizontally,versus resistance in ohms, vertically, for contacts having an 800 Athick 3 weight percent Be in Au layer, and a 2100 A thick Au layer. Theopen circles represent contacts with a 100 A thick Pd layer, and thesolid circles represent contacts in which a Pd layer was not present.The contacts were alloyed for 10 minutes. The contact resistances are aminimum between 400 and 440 degrees C. without the Pd layer. With the Pdlayer present, temperatures equal to or above 375 degrees C. may beused.

It is hypothesized that lower heat treating temperatures can be usedwith the palladium layer because the palladium layer traps outdiffusingphosphorous and forms intermetallic palladium-phosphorous compounds. Theresistance obtained by this scheme is generally slightly greater thanthat obtained without the palladium layer. However, the ohmic contactcan be formed adequately, i.e., with an acceptably small resistance, ata temperature as low as 375 degrees C. compared to the approximately 400degrees C. needed if the palladium layer is not present. At 375 and 400degrees C., the resistances are approximately 10 and 5 ohms,respectively.

FIG. 3 plots alloying time in minutes, horizontally, versus resistancein ohms, vertically, for contacts having an 800 A 3 weight percent Be inAu layer and a 2100 A Au layer. The contacts were alloyed at 420 degreesC. The contact resistances are a minimum for alloying times between 5and 10 minutes. Heat treating times outside this range lead to higherresistances, especially for shorter times. Longer times are notpreferred because of the increased possibility that undesiredintermetallic compounds may be formed in addition to the possibility ofInP decomposition.

Example: A double heterostructure InP/InGaAsP/InP light emitting diodewas grown by liquid phase epitaxy on a [100] oriented n-type InPsubstrate and consisted of a buffer layer approximately 2 microns thickSn doped, (n=10¹⁸ cm⁻³), a 1-micron thick active InGaAsP layer (n=2×10¹⁶cm⁻³), and a Zn-doped (n=10¹⁸ cm⁻³) p-type InP layer having thickness of1.5 microns. An ohmic contact was made to the p-type layer as describedabove. The contact was a 50 micron dot. The contact to the n-type layerwas a horseshoe-shaped sandwich of Au-Sn-Au about 5000 A thick. At acurrent of 60 mA and a forward voltage of approximately 1.5 eV, andpower emitted into the air was approximately 3 mW. This corresponds to apower conversion efficiency of approximately 3 percent. The upper limitof specific contact resistance was 7.8×10⁻⁵ ohm.cm. This value isapproximately two orders of magnitude lower than specific resistancespreviously reported for Au-Zn contacts to InP.

We claim:
 1. A method for making an ohmic contact to a semiconductordevice comprising a p-type semiconductor material selected from thegroup consisting of InP and InGaAsP comprising the steps of:depositingberyllium-gold on said semiconductor material; depositing gold on saidberyllium-gold; and heat treating said device at a temperature less than440 degrees C. for a residence time of at least 1 minute.
 2. A method asrecited in claim 1 in which said heat treating is in an inert orreducing atmosphere.
 3. A method as recited in claim 1 or 2 in whichsaid semiconductor is InP.
 4. A method as recited in claim 3 in whichsaid beryllium-gold layer has a beryllium content, by weight, between 1percent and 3 percent.
 5. A method as recited in claim 4 in which saidberyllium-gold layer is greater than 600 A and less than 1000 A.
 6. Amethod as recited in claim 5 in which said beryllium-gold layer isapproximately 800 A thick.
 7. A method as recited in claim 1 in whichsaid gold layer is at least 2100 A thick.
 8. A method as recited inclaim 5 in which said heat treating temperature is between 400 and 440degrees C.
 9. A method as recited in claim 8 in which said heat treatingis for approximately 10 minutes.
 10. A method as recited in claim 1further comprising the step of depositing a palladium layerapproximately 100 A thick above said InP layer, said palladium layerbeing deposited prior to said beryllium-gold layer.
 11. A method asrecited in claim 10 in which said heat treating is at a temperature lessthan 420 degrees C.
 12. A method as recited in claim 11 in which saidtemperature is approximately 400 degrees C.
 13. A method as recited inclaim 12 in which said heat treating is for approximately 10 minutes.14. A method as recited in claim 12 or 13 in which said device is alight emitting diode.
 15. A method as recited in claim 5 in which saidgold layer is at least 2100 Angstrom thick.